(i) Field of the Invention
The present invention mainly relates to pencil leads for mechanical pencils and wood-cased pencils. More specifically, it relates to color pencil leads which have excellent light resistance and weathering resistance and which always hold a sharp color and which can be easily erased by an eraser while keeping strength as calcined pencil leads, and it also relates to a method of manufacturing the color pencil leads.
(ii) Description of the Related Art
Conventional pencil leads can be roughly classified into four kinds of pencil leads which can be obtained by four different manufacturing methods, but any kind of pencil leads have some drawbacks.
(1) The pencil leads obtained by kneading, molding and then calcining a material comprising, as main components, graphite as a colorant, an organic substance and/or clay as a binder.
The pencil leads obtained by this method are known as pencil leads which have practical strength enough to be used in mechanical pencils and which can be erased by an eraser. However, this method has a drawback that in view of the employment of graphite and the color of the calcined carbon substance, it is impossible to manufacture the leads other than the black leads.
(2) The non-calcined pencil leads obtained by kneading and then molding a wax, a binder, a filler, a pigment and the like.
According to this method, many kinds of organic pigments can be used, and the color pencil leads which have light resistance, weathering resistance and a sharp color can be obtained.
However, in the non-calcined pencil leads obtained by this method, the binding of the respective components depends upon the binding power of the organic binder, and therefore the strength of the pencil leads is insufficient to use in the mechanical pencils, and since the wax is included, lines drawn by these pencil leads cannot be erased by an eraser.
(3) The non-calcined pencil leads obtained by kneading and molding a water/oil repellent substance, a binder, a filler, a pigment and the like, and then impregnating open cells formed around the water/oil repellent substance with an oil or the like.
According to this preparation method, the color pencil leads which can be erased by an eraser as easily as the pencil leads of the above-mentioned (1) can be prepared in the case that an oil or the like which is liquid at ordinary temperature is selected as the oil for the impregnation. However, when it is attempted to prepare the color pencil leads having a sharp hue by using an organic pigment, the usable binders are limited to organic polymeric substances owing to the heat resistance of the pigment. In consequence, the strength of the obtained pencil leads is insufficient to use in the mechanical pencils or the like.
(4) The pencil leads obtained by kneading, molding and then calcining boron nitride, clay, an organic substance and the like to obtain light-colored or white porous pencil leads, and then impregnating them with a dye-containing ink.
Lines drawn by the pencil leads obtained by this method can be erased by an eraser. However, since a pigment which has the sharp hue and a high heat-resistant temperature and which is harmless to humans and usable in the pencil leads is not present, the dye with which the calcined pencil leads can be impregnated is used as a colorant. Thus, the pencil leads obtained by this method are noticeably poorer in light resistance and weathering resistance as compared with the pigment leads of the above-mentioned (2) and (3). If the pencil leads having large cells therein are formed, it is possible to impregnate them with a pigment-dispersing ink, but in the pencil leads obtained in such a manner, the strength is low, so that the pencil leads cannot be used for the mechanical pencils.